The Vapor-compression refrigeration cycle (also referred to as Direct Expansion or DX) is the most widely used refrigeration method for storage space conditioning for perishable products and heating ventilation and air-conditioning applications. A simple DX refrigeration system 10 is represented in FIG. 1.
DX systems achieve a refrigeration effect by using a compressor 12 to compress a refrigerant such that the discharge pressure is greater than the corresponding Saturated Condensing Temperature (SCT), thereby causing the refrigerant at the outlet of a condenser 14 to enter a subcooled liquid state. The subcooled liquid is supplied to an expansion device 16 at discharge pressure and a temperature corresponding to a subcooled state of the refrigerant such that the refrigerant enters the expansion device 16 in a fully liquid state. The outlet of the expansion device 16 is at compressor suction pressure causing the refrigerant to vaporize and achieve the temperature corresponding to the pressure at the outlet of the expansion device 16 or, as depicted in FIG. 1, saturated suction temperature (SST). As the refrigerant vaporizes, heat energy is absorbed by the refrigerant via an evaporator 18 causing the refrigerant to enter a superheated vapor state before returning to the compressor 12 where the refrigerant is compressed and discharged at an elevated pressure and the cycle continues.
The amount of heat energy to be absorbed by the refrigerant to achieve the required evaporator temperature is referred to as heat load or simply load. This transfer of heat energy at the evaporator 18 is expressed as Q={dot over (m)}Δh where Q is Btu/hr, {dot over (m)} is mass flow of the refrigerant and Δh is the change in enthalpy of the refrigerant.
FIG. 2 shows a schematic diagram of another conventional vapor compression refrigeration system 10′. The system 10′ utilizes multiple compressors 12a-12n in a parallel configuration so as to provide varying amounts of refrigeration capacity in response to variations in load. Multiple evaporators 18a-18n are connected to a common compressor suction line and a remote outdoor condenser 14 is connected to the common compressor discharge.
Seasonal changes (e.g., ambient air temperature) can affect the SCT, and the amount of liquid refrigerant in the condenser 14 or one or more evaporators 18a-18n in the system entering a defrosting period, thereby reducing the amount of refrigerant circulating in the system 10′. To compensate for such seasonal changes, the system 10′ can include a refrigerant receiver 20 (also referred to as a receiver or a receiver vessel). The refrigerant receiver 20 allows sufficient refrigerant to be placed in the system 10′ to account for low outdoor ambient conditions when a substantial portion of the refrigerant will reside in the condenser 14, and high outdoor ambient conditions when excess refrigerant will reside in the receiver 20.
The system 10′ shown in FIG. 2 also can include a plurality of expansion valves 16a-16n. In order for the system to operate, a minimum pressure differential (ΔP) should exist across the expansion valves 16a-16n. During periods of low outdoor ambient temperatures the SCT will decrease to a level where the corresponding Saturated Condensing Pressure (SCP) will decrease to a pressure that no longer provides the expansion valves 16a-16n with sufficient pressure differential to operate. It is common practice to place a valve in the condenser outlet piping (Condenser Pressure Control Valve 22) to hold back liquid refrigerant in the condenser 14 during low outdoor ambient conditions to maintain a pressure adequate for proper operation of the expansion valves 16a-16n as the condensing pressure is approximately equal to the inlet pressure of the expansion valves 16a-16n. This decreases the effective surface area of the condenser which in turn raises the pressure at the inlet of the expansion valves 16a-16n. Such mechanical solution, while effective to maintain system operation, operates on a fixed pressure setting set by the installer.
During periods of exceptionally low ambient temperatures and/or low load conditions, low system refrigerant charge, etc. the mechanical limitations of the Condenser Pressure Control Valve 22 can allow the expansion valve inlet pressure to decrease below operational pressures. In order to prevent this condition from occurring, a close on rise of outlet pressure valve 24 (Receiver Pressure Regulating Valve) is provided to bypass the condenser 14 and pressurize the receiver 20 with compressor discharge vapor, thereby raising the inlet pressure to the expansion valves 16a-16n to a safe operating pressure (see system 10″ in FIG. 3). The valve 24 operates on a fixed value as set by the installer and is typically set to maintain a receiver pressure at a value lower than the Condenser Pressure Control Valve 22 setting.
FIG. 4 depicts a system 10′″ that includes a close on rise of outlet pressure valve (Liquid Pressure Regulating Valve) 26 to maintain a constant inlet pressure to the expansion valves 16a-16n, regardless of receiver pressure, as long as receiver pressure is above the Liquid Pressure Regulating Valve outlet pressure setting. This practice allows more accurate sizing of the expansion valve 16a-16n and consistent operation as the expansion valve capacity varies with inlet pressure.